Electroplating is a well known process by which a base material, such as a metal or plastic, is plated with a metal for decorative or protective purposes. The process is more formally defined by the ASTM (American Society for Testing and Materials) as the electrodeposition of an adherent metallic coating upon an electrode for the purpose of securing a surface with properties or dimensions different from those of the basis metal. ASTM B 374.
An electroplating process generally involves making the article to be plated cathodic in a solution of a salt of the coating metal. The anode may either be soluble (as with nickel, NI, and copper, Cu) and consist of the same metal, or it may be inert (such as with chromium, Cr).
Chromium plating is a distinct discipline within the field of electroplating. Chromium is almost always used as the final finish for nickel and copper-nickel decorative-protective systems. Chromium possesses the qualities of good corrosion resistance and resists wear and scratching. Among the applications for chromium plating is automotive exterior trim. The ASTM in specification B 456 rates this application as a very severe service condition, SC 4.
Decorative chromium is almost always applied over undercoatings of nickel or copper-nickel. A copper plating applied directly to the substrate provides a layer with relatively smooth or uniform surface characteristics. Nickel provides good adhesion for chromium and is quite corrosion-resistant. Nickel can be plated mat, semibright or fully bright. The nickel plate in a nickel-copper chromium or nickel-chromium composite has conventionally provided the main corrosion protection to the substrate. When using multiple layers of nickel in chromium plating, the first layer is normally a semibright nickel, and the second layer is of a bright nickel to provide a more lustrous adhesion surface for the chromium deposit.
There are a number of experience factors characteristic of chromium plating which must be considered in its practice. One factor is that the throwing power and the covering power of chromic acid-plating baths are among the poorest of any commercially used plating solution. In chromic acid-plating baths the current efficiency rises as the current density increases, at least within limits, and this phenomenon promotes non-uniform metal distribution on the plated article.
Another prominent factor is corrosion caused by dissimilar metal contact. By the very nature of electroplating metal coatings there is necessarily dissimilar metal contact. When an electrolyte, typically contaminated moisture, is available to complete the circuit corrosion will occur. As corrosion begins it progresses locally and forms a blister or cell until it reaches the substrate. This condition invites further corrosion and lateral spread from the site of the initial cell.
One technique for minimizing the deleterious effects of corrosion is to make the outer chromium layer microdiscontinuous to diffuse the galvanic action over the surface of the plated article. Microdiscontinuity can be achieved by making the chromium layer microcracked or microporous by known processes. This minimizes cell concentration and causes a slow, general corrosion of the surface rather than gross corrosion at a few spots or cells. However, even though microdiscontinuity tends to spread or diffuse the galvanic action, eventually the composite effect is to create an undesirable gradual dulling of the chromium plated surface.
Another factor, is that nickel is a relatively expensive plating material, and controlling its thicknesses to within prescribed dimensional limits introduces complexity and cost to the overall plating process.